Engineering strategies for peripheral nerve repair

Use of adipose-derived stem cells to fabricate scaffoldless tissue-engineered neural conduits in vitro

Peripheral nerve injuries resulting from trauma or disease often necessitate surgical intervention. Although the gold standard for such repairs uses nerve autografts, alternatives that do not require invasive harvesting of autologous nerve tissues are currently being designed and evaluated. We previously established the use of scaffoldless engineered neural conduits (ENCs) fabricated from primary cells as one such alternative in sciatic nerve repair in rats [Baltich et al. (2010) In Vitro Cell Dev Biol Anim 46(5):438-444]. The present study establishes protocols for fabricating neural conduits from adipose-derived stem cells (ASCs) differentiated to either a fibroblast or neural lineage and co-cultured into a three-dimensional (3-D) scaffoldless tissue-ENC. Addition of ascorbic acid-2-phosphate and fibroblast growth factor (FGF)-2 to the medium induced and differentiated ASCs to a fibroblast lineage in more than 90% of the cell population, as confirmed by collagen I expression. ASC-differentiated fibroblasts formed monolayers, delaminated, and formed 3-D conduits. Neurospheres were formed by culturing ASCs on non-adherent surfaces in serum-free neurobasal medium with the addition of epidermal growth factor (EGF) and FGF-2. The addition of 10 ng EGF and 10 ng FGF-2 produced larger and more numerous neurospheres than treatments of lower EGF and FGF-2 concentrations. Subsequent differentiation to glial-like cells was confirmed by the expression of S100. ASC-derived fibroblast monolayers and neurospheres were co-cultured to fabricate a 3-D scaffoldless tissue-ENC. Their nerve-like structure and incorporation of glial-like cells, which would associate with regenerating axons, may make these novel, stem cell-derived neural conduits an efficacious technology for repairing critical gaps following peripheral nerve injury.

Joint use of a chitosan/PLGA scaffold and MSCs to bridge an extra large gap in dog sciatic nerve

BACKGROUND

Tissue-engineered nerve grafts (TENGs) constitute a promising alternative to nerve autografts that are recognized as the gold standard for surgical repair of peripheral nerve gaps.

OBJECTIVE

To investigate the feasibility of using TENGs for bridging extra large peripheral nerve gaps in large animals.

METHODS

TENGs were constructed by incorporating autologous bone marrow mesenchymal stem cells (MSCs) into a neural scaffold that consisted of a chitosan conduit inserted with poly(lactic-co-glycolic acid) (PLGA) fibers. A 60-mm-long sciatic nerve gap in dogs was bridged by TENGs, chitosan/PLGA scaffolds, or nerve autografts. At 12 months postsurgery, behavioral analysis, electrophysiology, retrograde fluorogold tracing, and histological examination were performed.

RESULTS

The outcomes of TENGs were similar to those of autografts and better than those of scaffolds alone.

CONCLUSION

Introduction of autologous MSCs to a chitosan/PLGA scaffold improved the repair and rehabilitation of a large gap after peripheral nerve injury in dogs. Autologous MSCs may be a source of support cells for neural tissue engineering.

Reconstruction of digital nerves with collagen conduits

PURPOSE

Digital nerve reconstruction with a biodegradable conduit offers the advantage of providing nerve reconstruction while providing a desirable environment for nerve regeneration. Many conduit materials have been investigated, but there have been few reports of human clinical trials of purified type I bovine collagen conduits.

METHODS

We report a prospective study of 22 isolated digital nerve lacerations in 19 patients reconstructed with a bioabsorbable collagen conduit. The average nerve gap measured 12 mm. An independent observer performed the postoperative evaluation, noting the return of protective sensation, static 2-point discrimination, and moving 2-point discrimination, and recording the patient's pain level using a visual analog scale. Minimal follow-up was 12 months and mean follow-up was 20 months after surgery.

RESULTS

All patients recovered protective sensation. The mean moving 2-point discrimination and static 2-point discrimination measured 5.0 and 5.2 mm, respectively, for those with measurable recovery at final follow-up visit. Excellent results were achieved in 13 of 22 digits, good results in 3 of 22 digits, and fair results in 6 of 22 digits, and there were no poor results. Reported pain scores at the last postoperative visit were measured universally as 0 on the visual analog scale.

CONCLUSIONS

Our data suggest that collagen conduits offer an effective method of reconstruction for digital nerve lacerations. This study confirms that collagen conduits reliably provide a repair that restores nerve function for nerve gaps measuring less than 2 cm.

A prospective randomized study comparing woven polyglycolic acid and autogenous vein conduits for reconstruction of digital nerve gaps

PURPOSE

The optimal management of a nerve gap within the fingers remains an unanswered question in hand surgery. The purpose of this study was to compare the sensory recovery, cost, and complication profile of digital nerve repair using autogenous vein and polyglycolic acid conduits.

METHODS

We enrolled patients undergoing repair of digital nerve injuries with gaps precluding primary repair. The minimum gap that was found to preclude primary repair was 4 mm. Each nerve repair was randomized to the type of nerve repair with either a woven polyglycolic acid conduit or autogenous vein. Time required for repair was recorded. We performed sensory testing, consisting of static and moving 2-point discrimination, at 6 and 12 months after repair. We compared patient factors between the 2 groups using chi-square and Student's t-test. We compared sensory recovery between the 2 groups at each time point using Student's t-test and compared time and cost of repair.

RESULTS

We enrolled 42 patients with 76 nerve repairs. Of these, 37 patients (representing 68 repairs) underwent sensory evaluation at the 6-month time point. The median age in this group was 35 years. We repaired 36 nerves with synthetic conduit and 32 with vein. Nerve gaps ranged from 4 to 25 mm (mean, 10 mm). Study groups were not significantly different regarding age, time to repair, gap length, medical history, smoking history, or worker's compensation status. Time to harvest the vein was longer but the average cost of materials and surgery in the vein group was $1,220, compared with $1,269 for synthetic conduit repairs. These differences were not statistically significant. Mean static and moving 2-point discrimination at 6 months for the synthetic conduit group were 8.3 ± 2.0 and 6.6 ± 2.3, respectively, compared with 8.5 ± 1.8 and 7.1 ± 2.2 for the vein group. Values at 12 months for the synthetic conduit group were 7.5 ± 1.9 and 5.6 ± 2.2, compared with 7.6 ± 2.6 and 6.6 ± 2.9 for the vein group. These differences were not statistically significant. Smokers and worker's compensation patients had a worse sensory recovery at 12 months postrepair. There were 2 extrusions in the synthetic conduit group requiring reoperation; however, the difference in extrusion rate was not found to be statistically significant.

CONCLUSIONS

Sensory recovery after digital nerve reconstruction with autogenous vein conduit was equivalent to that using polyglycolic acid conduit, with a similar cost profile and fewer postoperative complications.

Regeneration and repair of peripheral nerves with different biomaterials: review

Peripheral nerve injury may cause gaps between the nerve stumps. Axonal proliferation in nerve conduits is limited to 10-15 mm. Most of the supportive research has been done on rat or mouse models which are different from humans. Herein we review autografts and biomaterials which are commonly used for nerve gap repair and their respective outcomes. Nerve autografting has been the first choice for repairing peripheral nerve gaps. However, it has been demonstrated experimentally that tissue engineered tubes can also permit lead to effective nerve repair over gaps longer than 4 cm repair that was previously thought to be restorable by means of nerve graft only. All of the discoveries in the nerve armamentarium are making their way into the clinic, where they are, showing great potential for improving both the extent and rate of functional recovery compared with alternative nerve guides.

Construction of tissue engineered nerve grafts and their application in peripheral nerve regeneration

Surgical repair of severe peripheral nerve injuries represents not only a pressing medical need, but also a great clinical challenge. Autologous nerve grafting remains a golden standard for bridging an extended gap in transected nerves. The formidable limitations related to this approach, however, have evoked the development of tissue engineered nerve grafts as a promising alternative to autologous nerve grafts. A tissue engineered nerve graft is typically constructed through a combination of a neural scaffold and a variety of cellular and molecular components. The initial and basic structure of the neural scaffold that serves to provide mechanical guidance and optimal environment for nerve regeneration was a single hollow nerve guidance conduit. Later there have been several improvements to the basic structure, especially introduction of physical fillers into the lumen of a hollow nerve guidance conduit. Up to now, a diverse array of biomaterials, either of natural or of synthetic origin, together with well-defined fabrication techniques, has been employed to prepare neural scaffolds with different structures and properties. Meanwhile different types of support cells and/or growth factors have been incorporated into the neural scaffold, producing unique biochemical effects on nerve regeneration and function restoration. This review attempts to summarize different nerve grafts used for peripheral nerve repair, to highlight various basic components of tissue engineered nerve grafts in terms of their structures, features, and nerve regeneration-promoting actions, and finally to discuss current clinical applications and future perspectives of tissue engineered nerve grafts.

Novel degradable co-polymers of polypyrrole support cell proliferation and enhance neurite out-growth with electrical stimulation

Synthetic polymers such as polypyrrole (PPy) are gaining significance in neural studies because of their conductive properties. We evaluated two novel biodegradable block co-polymers of PPy with poly(epsilon-caprolactone) (PCL) and poly(ethyl cyanoacrylate) (PECA) for nerve regeneration applications. PPy-PCL and PPy-PECA co-polymers can be processed from solvent-based colloidal dispersions and have essentially the same or greater conductivity (32 S/cm for PPy-PCL, 19 S/cm for PPy-PECA) compared to the PPy homo-polymer (22 S/cm). The PPy portions of the co-polymers permit electrical stimulation whereas the PCL or PECA blocks enable degradation by hydrolysis. For in vitro tests, films were prepared on polycarbonate sheets by air brushing layers of dispersions and pressing the films. We characterized the films for hydrolytic degradation, electrical conductivity, cell proliferation and neurite extension. The co-polymers were sufficient to carry out electrical stimulation of cells without the requirement of a metallic conductor underneath the co-polymer film. In vitro electrical stimulation of PPy-PCL significantly increased the number of PC12 cells bearing neurites compared to unstimulated PPy-PCL. For in vivo experiments, the PPy co-polymers were coated onto the inner walls of nerve guidance channels (NGCs) made of the commercially available non-conducting biodegradable polymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-HV). The NGCs were implanted in a 10 mm defect made in the sciatic nerve of rats, and harvested after 8 weeks. Histological staining showed axonal growth. The studies indicated that these new conducting degradable biomaterials have good biocompatibility and support proliferation and growth of PC12 cells in vitro (with and without electrical stimulation) and neurons in vivo (without electrical stimulation).

Development of a scaffoldless three-dimensional engineered nerve using a nerve-fibroblast co-culture

Nerve grafts are often required to replace tissue damaged by disease, surgery, or extensive trauma. Limitations such as graft availability, donor site morbidity, and immune rejection have led investigators to develop strategies to engineer nerve tissue. The goal of this study was to fabricate a scaffoldless three-dimensional (3D) nerve construct using a co-culture of fetal nerve cells with a fibroblast monolayer and allow the co-culture to remodel into a 3D construct with an external fibroblast layer and an internal core of interconnected neuronal cells. Primary fibroblasts were seeded on laminin-coated plates and allowed to form a confluent monolayer. Neural cells isolated from E-15 spinal cords were seeded on top of the fibroblast monolayer and allowed to form a networked monolayer across the monolayer of fibroblasts. Media shifts initiated contraction of the fibroblast monolayer and a remodeling of the co-culture into a 3D construct held statically in place by the two constraint pins. Immunohistochemistry using S100 (Schwann cell), beta3-tubulin, DAPI, and collagen I indicated an inner core of nerve cells surrounded by an external layer of fibroblasts. Conduction velocities of the 3D nerve and control (fibroblast-only) constructs were measured in vitro and compared to in vivo measures of neonatal sciatic nerve. The conduction velocities of the nerve constructs were comparable to 24-d-old neonatal nerve. The presence of Schwann cells and the ability to conduct neuronal signals in vitro suggest the scaffoldless 3D nerve constructs will be a viable option for nerve repair.

Early clinical outcomes with the use of decellularized nerve allograft for repair of sensory defects within the hand

Nerve conduits have become an established option for repair of sensory deficits of up to 2 cm. More recently, decellularized nerve allograft has also been advocated as an option for nerve repair; however, no clinical studies have examined its efficacy for the treatment of sensory nerve defects. The aim of this study was to examine our early experience with the use of decellularized nerve allograft for repair of segmental nerve defects within the hand and fingers. From July 2007 to March 2008, seven patients who had ten nerve gaps were treated surgically using decellularized nerve allograft. Eight digital and two dorsal sensory nerves were repaired. The etiologies of the nerve defects were traumatic nerve transection in eight defects and neuroma resection and reconstruction in two defects. All of the affected nerves were pure sensory fibers. Functional recovery was evaluated by blinded hand therapist using moving and static two point discrimination tests. Implantation sites were also evaluated for any signs of infection, rejection, or graft extrusion. There were five men and two women with a mean age of 44 years (range 23-65). Mean nerve graft length was 2.23 cm with a range of 0.5-3 cm. Mean follow up time was 9 months (range 5-12). Average two point discrimination was 4.4 mm moving and 5.5 mm static at last recorded follow-up. There were no wound infections observed around the graft material and sensory improvement was observed in all of the patients despite this short-term follow-up. Re-exploration of two fingers was required for flexor tendon rupture in one and flexor tendon tenolysis in the other. In both cases, the nerve allograft was visualized and appeared well incorporated in the repair site. Decellularized nerve allografts were capable of returning adequate sensation in nerve defects ranging from 0.5 to 3 cm. There were no cases of infection or rejection. Decellularized nerve allograft may provide an option for segmental nerve gaps beyond 2 cm. Randomized comparative studies will be required to determine efficacy in comparison to collagen conduits or nerve autograft.

Siatic nerve regeneration in rats stimulated by fibrin glue containing nerve growth factor: an experimental study

OBJECTIVE

To study the effects of fibrin sealant containing nerve growth factor on the peripheral nerve regeneration.

STUDY DESIGN

A four-group experimental design with repeated measures on one factor was used.

METHODS

Fibrin glue (FG) containing NGF was injected into the site of end-and-end sutured peripheral nerve (sciatic nerve) (group I: NGF+FG), meanwhile three control groups were set-up: group II (NGF), group III (FG), and group IV (normal saline). Observation to the function and morphology of the sciatic nerve was carried out at the end of 4, 8, 12 weeks postoperation. Data were analyzed using ANOVA, with the appropriate post hoc between-groups comparison test.

RESULTS

Electrophysiological testing. The NAP and NCV of group I (NGF+FG) were greater than those of group II (NGF), group III (FG), or group IV (normal saline) (p<0.05). Sciatic functional index (SFI). It began to ameliorate at 4 weeks postoperation and SFI increased as time went on. And the SFI in group I (NGF+FG) was better than those in group II (NGF), group III (FG), or group IV (normal saline) (p<0.05). Morphology. In the MGF-stained sections, dissociated myelin debris was less and regenerated nerve fibres were in larger quantities in group I (NGF+FG) than in other groups. In the HE-stained sections, regenerated nerve fibres distal to anastomosis significantly increased, and axon and myelin had a clearer outline in group I (NGF+FG) than in other groups. Electron microscopy indicated that the regenerated nerve fibres were more mature and the development of the axons was greater in group I than in other control groups.

CONCLUSIONS

FG can be used as carrier of exogenous NGF, and they can provide synergistic effects for the peripheral nerve regeneration after the integration of the two.

Implantation of neural stem cells embedded in hyaluronic acid and collagen composite conduit promotes regeneration in a rabbit facial nerve injury model

The implantation of neural stem cells (NSCs) in artificial scaffolds for peripheral nerve injuries draws much attention. NSCs were ex-vivo expanded in hyaluronic acid (HA)-collagen composite with neurotrophin-3, and BrdU-labeled NSCs conduit was implanted onto the ends of the transected facial nerve of rabbits. Electromyography demonstrated a progressive decrease of current threshold and increase of voltage amplitude in de-innervated rabbits after implantation for one, four, eight and 12 weeks compared to readouts derived from animals prior to nerve transection. The most remarkable improvement, observed using Electrophysiology, was of de-innervated rabbits implanted with NSCs conduit as opposed to de-innervated counterparts with and without the implantation of HA-collagen, NSCs and HA-collagen, and HA-collagen and neurotrophin-3. Histological examination displayed no nerve fiber in tissue sections of de-innervated rabbits. The arrangement and S-100 immunoreactivity of nerve fibers in the tissue sections of normal rabbits and injured rabbits after implantation of NSCs scaffold for 12 weeks were similar, whereas disorderly arranged minifascicles of various sizes were noted in the other three arms. BrdU+ cells were detected at 12 weeks post-implantation. Data suggested that NSCs embedded in HA-collagen biomaterial could facilitate re-innervations of damaged facial nerve and the artificial conduit of NSCs might offer a potential treatment modality to peripheral nerve injuries.

Nerve repair: experimental and clinical evaluation of biodegradable artificial nerve guides

Several methods have been used for bridging nerve gaps. Much of the focus in nerve repair of peripheral nerves has focussed on creating either natural or synthetic tubular nerve guidance channels, as an alternative to nerve autografts. These conduits act to guide axons sprouting from the regenerating nerve end, provide a conduit for diffusion of neurotrophic and neurotropic factors secreted by the injured nerve stump, as well as help protect against infiltration of fibrous tissue. Among the conduits that have been studied are autogenous veins, arteries, mesothelial chambers, synthetic tubes, collagen tubes, amnion tubes, cardiac and skeletal muscle, and silicon tubes. This paper briefly reviews major studies in which bioabsorbable nerve guides were used for peripheral nerve repair, with a particular emphasis on polymeric guidance channels, in an effort to evaluate their use, their ability to support or enhance nerve regeneration and any potential problems.

Repair of lacerated peripheral nerves with nerve conduits

Peripheral nerve lesions are relatively common injuries encountered by hand surgeons. These injuries are notorious for causing significant and potentially long-standing impairment to hand function. Numerous surgical techniques with varying degrees of success have been described to treat this injury. The evolution of peripheral nerve repair has led to the development of the nerve conduit, a surgical technique that functionally bridges the gap between transected nerves. We discuss a brief history and evolution of nerve conduits and offer our preferred technique for peripheral nerve repair with a collagen nerve conduit. In addition, we offer case studies and postoperative rehabilitation goals and present early results associated with this type of repair.

Electrospun nanofibers immobilized with collagen for neural stem cells culture

Fibrous mats via electrospinning have been widely applied in tissue engineering. In this work, nanofibers were prepared via electrospinning from polymer with different content of carboxyl groups. A natural material, collagen, was then immobilized onto the nanofiber surface by N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC)/N-Hydroxysuccinimide (NHS) activation process. It was found that the immobilization degree of collagen could be facilely modulated. The obtained collagen-modified nanofibers were used for neural stem cells culture, and unmodified nanofibers were used as a control. Results indicated that the modification of collagen could enhance the attachment and viability of the cultured neural stem cells.

Tissue engineering in head and neck reconstructive surgery: what type of tissue do we need?

Craniofacial tissue loss due to congenital defects, disease or injury is a major clinical problem. The head and neck region is composed of several tissues. The most prevalent method of reconstruction is autologous grafting. Often, there is insufficient host tissue for adequate repair of the defect side, and extensive donor site morbidity may result from the secondary surgical procedure. The field of tissue engineering has the potential to create functional replacements for damaged or pathologic tissues.

Nanofunctionalisation for the treatment of peripheral nervous system injuries

A construct based on the electrostatic layer-by-layer self assembly technique has been fabricated, to be used as a tailored device to encourage nerve regeneration. A multilayered nanocoating composed by three precursor bilayers of cationic and anionic polyelectrolytes followed by bilayers of poly-D-lysine (PDL) and antibody specific to Transforming Growth Factor beta1 (anti-TGF-beta1) has been deposited on HYAFF 11. Initially the assembly process has been monitored by quartz crystal microbalance (QCM) in order to select the optimal working conditions for nanocoating deposition. Structural studies of the resulting multilayers confirmed stepwise deposition of anti-TGF-beta1 with an average layer thickness of 2.6 nm and an average layer mass of 117 ng. Atomic Force Microscopy has been used to characterize multilayer uniformity. Finally, the immunological activity of the multilayered structure has been assessed. The results show that anti-TGF-beta1 can be included in its active form in a predetermined multilayered structure onto HYAFF11 with quantitative control of layer thickness and weight, providing a high potential tool in tissue engineering.

Evaluation of cellular organization and axonal regeneration through linear PLA foam implants in acute and chronic spinal cord injury

There are few studies of neural implants in spinal cord injury (SCI) focused on supporting directed axon growth. In this study, we fabricated a macroporous poly (lactic acid) (PLA) foam with oriented inner channels. Amorphous foam without linear channels served as a control in an acute SCI injury model, and the effectiveness of foam with linear channels was further investigated in a chronic SCI model. Implants were placed into a 2 mm hemisection lesion cavity at the T8 spinal cord level in adult rats. Two weeks post-implantation, tissue sections including the implants were examined using antibodies against GFAP, p75, ED-1, laminin, GAP-43, and CGRP. Foam implants were well-integrated with the host spinal cord. In linear foams, numerous DAPI-stained cells were found within the inner channels. Schwann cells but not astrocytes had migrated within the channels. Intense laminin staining was observed throughout the extracellular matrix substrate. GAP-43- and CGRP-positive axons grew through the implants following the linear channels. In the amorphous control foams, DAPI staining distributed evenly through the pores. However, the growth of GAP-43 or CGRP-positive axons was misguided and impeded at the entrance area of the foam. Higher numbers of GAP-43 and CGRP-positive axons grew into linear foam implants after chronic SCI than acute SCI. These results suggest the potential application of linear foam implants in cell and axon guidance for SCI repair, especially for chronic SCI.

Electrospun nanofibres: Biomedical applications

Synthetic and semi-synthetic polymeric materials were originally developed for their durability and resistance to all forms of degradation, including biodegradation. Nanotechnology has the potential to revolutionize many sectors, including pharmaceuticals, information technology, medical devices, materials science, chemicals, and energy. Nanofibres provide a connection between the nanoscale world and the macroscale world, since their diameters are in the range of 1 to 100 nanometres and several metres in length. Therefore, the current emphasis of research is to exploit such properties and focus on determining appropriate conditions for electrospinning various polymers and biopolymers for eventual applications including: multifunctional membranes; biomedical structural elements (scaffolds used in tissue engineering, wound dressing, drug delivery, artificial organs, vascular grafts); protective shields in specialty fabrics; and filter media for submicron particles in the separation industry. This paper reviews the research on recent biomedical applications of electrospun nanofibres.

Nerve growth factor expression by PLG-mediated lipofection

Biomaterials capable of efficient gene delivery provide a fundamental tool for basic and applied research models, such as promoting neural regeneration. We developed a system for the encapsulation and sustained release of plasmid DNA complexed with a cationic lipid and investigated their efficacy using in vitro models of neurite outgrowth. Sustained lipoplex release was obtained for up to 50 days, with rates controlled by the fabrication conditions. Released lipoplexes retained their activity, transfecting 48.2+/-8.3% of NIH3T3 cells with luciferase activity of 3.97x10(7)RLU/mg. Expression of nerve growth factor (NGF) was employed in two models of neurite outgrowth: PC12 and primary dorsal root ganglia (DRG) co-culture. Polymer-mediated lipofection of PC12 produced bioactive NGF, eliciting robust neurite outgrowth. An EGFP/NGF dual-expression vector identified transfected cells (GFP-positive) while neurite outgrowth verified NGF secretion. A co-culture model examined the ability of NGF secretion by an accessory cell population to stimulate DRG neurite outgrowth. Polymer-mediated transfection of HEK293T with an NGF-encoding plasmid induced outgrowth by DRG neurons. This system could be fabricated as implants or nerve guidance conduits to support cellular and tissue regeneration. Combining this physical support with the ability to locally express neurotrophic factors will potentiate regeneration in nerve injury and disease models.

Regeneration and repair of peripheral nerves

Posttraumatic nerve repair continues to be a major challenge in restorative medicine and microsurgery. Although progress has been made in surgical techniques over the last 30 years, functional recovery after a severe lesion of a major nerve trunk is often incomplete and often unsatisfactory. Functional recovery after surgical repair of mixed nerves is even more disappointing. Functional recovery after peripheral nerve lesion is dependent upon accurate regeneration of axons to their original target tissues. Thus, in order to enhance regeneration, a better understanding of the cellular and molecular biology of selective nerve regeneration is required. Schwann cells and their endoneurial extracellular matrix play pivotal roles in the selective promotion of motor and sensory axon regeneration. Knowledge of these mechanisms allows for the better development of biocompatible nerve grafting material.

Nerve conduits

The evolution of peripheral nerve repair is reviewed with respect to the development of the nerve conduit. The rationale and available scientific evidence to support the use of nerve conduits is presented. Therapy evaluation and treatment protocols for patients with peripheral nerve repairs with nerve conduits are detailed. The authors present clinical experience to date with 73 cases of peripheral nerves repaired with the NeuraGenR collagen nerve conduit.

Optimized acellular nerve graft is immunologically tolerated and supports regeneration

To replace the autologous graft as a clinical treatment of peripheral nerve injuries we developed an optimized acellular (OA) nerve graft that retains the extracellular structure of peripheral nerve tissue via an improved chemical decellularization treatment. The process removes cellular membranes from tissue, thus eliminating the antigens responsible for allograft rejection. In the present study, the immunogenicity and regenerative capacity of the OA grafts were tested. Histological examination of the levels of CD(8+) cells and macrophages that infiltrated the OA grafts suggested that the decellularization process averted cell-mediated rejection of the grafts. In a subsequent experiment, regeneration in OA grafts was compared with that in isografts (comparable to the clinical autograft) and two published acellular graft models. After 84 days, the axon density at the midpoints of OA grafts was statistically indistinguishable from that in isografts, 910% higher than in the thermally decellularized model described by Gulati (J. Neurosurg. 68, 117, 1988), and 401% higher than in the chemically decellularized model described by Sondell et al. (Brain Res. 795, 44, 1998). In summary, the results imply that OA grafts are immunologically tolerated and that the removal of cellular material and preservation of the matrix are beneficial for promoting regeneration through an acellular nerve graft.

Delivery systems for small molecule drugs, proteins, and DNA: the neuroscience/biomaterial interface

Manipulation of cellular processes in vivo by the delivery of drugs, proteins or DNA is of paramount importance to neuroscience research. Methods for the presentation of these molecules vary widely, including direct injection (either systemic or stereotactic), osmotic pump-mediated chronic delivery, or even implantation of cells engineered to indefinitely secrete a factor of interest. Biomaterial-based delivery systems represent an alternative to more traditional approaches, with the possibility of increased efficacy. Drug-releasing biomaterials, either as injectable microspheres or as three-dimensional implants, can deliver a molecule of interest (including small molecule drugs, biologically active proteins, or DNA) over a more prolonged period of time than by standard bolus injection, avoiding the need for repeated administration. Furthermore, sustained-release systems can maintain therapeutic concentrations at a target site, thus reducing the chance for toxicity. This review summarizes applications of polymer-based delivery of small molecule drugs, proteins, and DNA specifically relevant to neuroscience research. We detail the fabrication procedures for the polymeric systems and their utility in various experimental models. The biomaterial field offers unique experimental tools with downstream clinical application for the study and treatment of neurologic disease.

Functional and Morphological Assessment of a Standardized Rat Sciatic Nerve Crush Injury with a Non-Serrated Clamp

Peripheral nerve researchers frequently use the rat sciatic nerve crush as a model for axonotmesis. Unfortunately, studies from various research groups report results from different crush techniques and by using a variety of evaluation tools, making comparisons between studies difficult. The purpose of this investigation was to determine the sequence of functional and morphologic changes after an acute sciatic nerve crush injury with a non-serrated clamp, giving a final standardized pressure of p = 9 MPa. Functional recovery was evaluated using the sciatic functional index (SFI), the extensor postural thrust (EPT) and the withdrawal reflex latency (WRL), before injury, and then at weekly intervals until week 8 postoperatively. The rats were also evaluated preoperatively and at weeks 2, 4, and 8 by ankle kinematics, toe out angle (TOA), and gait-stance duration. In addition, the motor nerve conduction velocity (MNCV) and the gastrocnemius-soleus weight parameters were measured just before euthanasia. Finally, structural, ultrastructural and histomorphometric analyses were carried out on regenerated nerve fibers. At 8 weeks after the crush injury, a full functional recovery was predicted by SFI, EPT, TOA, and gait-stance duration, while all the other parameters were still recovering their original values. On the other hand, only two of the histomorphometric parameters of regenerated nerve fibers, namely myelin thickness/axon diameter ratio and fiber/axon diameter ratio, returned to normal values while all other parameters were significantly different from normal values. The employment of traditional methods of functional evaluation in conjunction with the modern techniques of computerized analysis of gait and histomorphometric analysis should thus be recommended for an overall assessment of recovery in the rat sciatic nerve crush model.

Fabrication of nano-structured porous PLLA scaffold intended for nerve tissue engineering

Nerve tissue engineering (NTE) is one of the most promising methods to restore central nerve systems in human health care. Three-dimensional distribution and growth of cells within the porous scaffold are of clinical significance for NTE. In this study, an attempt was made to develop porous polymeric nano-fibrous scaffold using a biodegradable poly(L-lactic acid) (PLLA) for in vitro culture of nerve stem cells (NSCs). The processing of PLLA scaffold has been carried out by liquid-liquid phase separation method. The physico-chemical properties of the scaffold were fully characterized by using differential scanning calorimetry and scanning electron microscopy. These results confirmed that the prepared scaffold is highly porous and fibrous with diameters down to nanometer scale. As our nano-structured PLLA scaffold mimics natural extracellular matrix, we have intended this biodegradable scaffold as cell carrier in NTE. The in vitro performance of NSCs seeded on nano-fibrous scaffold is addressed in this study. The cell cultural tests showed that the NSCs could differentiate on the nano-structured scaffold and the scaffold acted as a positive cue to support neurite outgrowth. These results suggested that the nano-structured porous PLLA scaffold is a potential cell carrier in NTE.

Use of Skeletal Muscle Tissue in Peripheral Nerve Repair: Review of the Literature

The management of peripheral nerve injury continues to be a major clinical challenge. The most widely used technique for bridging defects in peripheral nerves is the use of autologous nerve grafts. This technique, however, necessitates a donor nerve and corresponding deficit. Many alternative techniques have thus been developed. The use of skeletal muscle tissue as graft material for nerve repair is one example. The rationale regarding the use of the skeletal muscle tissue technique is the availability of a longitudinally oriented basal lamina and extracellular matrix components that direct and enhance regenerating nerve fibers. These factors provide superiority over other bridging methods as vein grafts or (non)degradable nerve conduits. The main disadvantages of this technique are the risk that nerve fibers can grow out of the muscle tissue during nerve regeneration, and that a donor site is necessary to harvest the muscle tissue. Despite publications on nerve conduits as an alternative for peripheral nerve repair, autologous nerve grafting is still the standard care for treatment of a nerve gap in the clinical situation; however, the use of the skeletal muscle tissue technique can be added to the surgeon's arsenal of peripheral nerve repair tools, especially for bridging short nerve defects or when traditional nerve autografts cannot be employed. This technique has been investigated both experimentally and clinically and, in this article, an overview of the literature on skeletal muscle grafts for bridging peripheral nerve defects is presented.

Residual function in peripheral nerve stumps of amputees: implications for neural control of artificial limbs

PURPOSE

It is not known whether motor and sensory pathways associated with a missing or denervated limb remain functionally intact over periods of many months or years after amputation or chronic peripheral nerve transection injury. We examined the extent to which activity on chronically severed motor nerve fibers could be controlled by human amputees and whether distally referred tactile and proprioceptive sensations could be induced by stimulation of sensory axons in the nerve stumps.

METHODS

Amputees undergoing elective stump procedures were invited to participate in this study. Longitudinal intrafascicular electrodes were threaded percutaneously and implanted in severed nerves of human amputees. The electrodes were interfaced to an amplifier and stimulator system controlled by a laptop computer. Electrophysiologic tests were conducted for 2 consecutive days after recovery from the surgery.

RESULTS

It was possible to record volitional motor nerve activity uniquely associated with missing limb movements. Electrical stimulation through the implanted electrodes elicited discrete, unitary, graded sensations of touch, joint movement, and position, referring to the missing limb.

CONCLUSIONS

These findings indicate that both central and peripheral motor and somatosensory pathways retain significant residual connectivity and function for many years after limb amputation. This implies that peripheral nerve interfaces could be used to provide amputees with prosthetic limbs that have more natural feel and control than is possible with current myoelectric and body-powered control systems.

Enhanced peripheral nerve regeneration through a poled bioresorbable poly(lactic-co-glycolic acid) guidance channel

In this study we investigated the effects of materials prepared with electrical poling on neurite outgrowth in vitro and nerve regeneration in vivo. Neuro-2a cells were seeded on poled and unpoled poly(lactic-co-glycolic) (PLGA) films and observed at time periods 24, 48 and 72 h post-seeding. The percentage of cells with neurites and the neurites per cell were quantified using light microscopy. At 48 and 72 h post-seeding, both the number of cells with neurites and the neurites per cell were significantly increased on the poled films compared to those on unpoled films. An established rat sciatic nerve model was used for in vivo studies to assess the effects of PLGA guides, poled for two different periods, on peripheral nerve regeneration. Guides were inserted in rats to bridge a 1.0 cm gap created in the right sciatic nerve. After four weeks, nerves regenerated through poled guides displayed a significant increase in conduction velocity and significantly increased numbers of axons across the guides, as compared to nerves regenerating through an unpoled guidance channel. Electrical poling was shown to promote neurite growth, axon regeneration and the conduction rate of the repaired nerve. We concluded that guides prepared with electrical poling enhance peripheral nerve regeneration.

Fabricating autologous tissue to engineer artificial nerve

This study reports on the successful fabrication of artificial nerves with tissue engineering methods. Schwann cells were cultured for 2 weeks, seeded on polyglactin 910 scaffolds, and biomembrane-coated with rat-tail glue and laminin. Observation of the scaffolds' adsorptivity to Schwann cells, and of the growth and migration of Schwann cells, was made using a light microscope, and by scanning and transmission electron microscopy. The Schwann cells were able to migrate and proliferate on the polyglactin 910 fiber. Schwann cells were well-distributed, and formed a Büngner band on which the Schwann cells produced more matrices. Schwann cells on the biomembrane also grew well. We investigated the role of the tissue engineering conduit guide in vivo, using an established rabbit peripheral nerve regeneration model. At 8 weeks, axonal regeneration was observed in the distal nerve stump. Adult Schwann cells can be produced on the coated fiber and the biomembrane. Three-dimensional scaffolds with Schwann cells had the basic characteristics of the artificial nerve. These findings will provide a practical method for fabricating tissue-engineered artificial nerve to repair long nerve defects.

Transplantation of olfactory ensheathing cells stimulates the collateral sprouting from axotomized adult rat facial motoneurons

Axon regrowth after CNS and PNS injury is only the first step toward complete functional recovery which depends largely on the specificity of the newly formed nerve-target projections. Since most of the studies involving the application of glial cells to the lesioned nervous system have focused primarily on the extent of neurite outgrowth, little is known regarding their effects on the accompanying processes of axonal sprouting and pathfinding. In this study, we analyzed the effects of transplanted olfactory ensheathing cells (OECs) on axonal sprouting of adult facial neurons by using triple fluorescent retrograde tracing and biometrical analysis of whisking behavior. We found that 2 months after facial nerve axotomy and immediate implantation of OECs in between both nerve stumps fixed in a silicon tube, the total number of labeled neurons was increased by about 100%, compared to animals with simple facial nerve suture or entubulation in an empty conduit. This change in the number of axon sprouts was not random. The highest increase in axon number was observed in the marginal mandibular branch, whereas no changes were detected in the zygomatic branch. This increased sprouting did not improve the whisking behavior as measured by biometric video analysis. Our results demonstrate that OECs are potent inducers of axonal sprouting in vivo. Hence OEC-filled nerve conduits may be a powerful tool to enforce regeneration of a peripheral nerve under adverse conditions, e.g., after long delay between injury and surgical repair. In mixed nerves, increased axonal sprouting will improve specificity since inappropriate nerve-target connections are pruned off during preferential motor innervation. In pure motor nerves, however, OEC-mediated axonal sprouting may result in polyneuronal innveration of target muscles.